This invention relates generally to electronic circuits and more particularly to an improved operational amplifier output stage.
The electronics industry is continually attempting to provide products with lower and lower power requirements. One standard method for achieving reduced power requirements is to use lower power supplies to the components. However, certain conventional components are unable to operate with these lower power supplies.
For example, in order to achieve high efficiency and high output current drive, conventional operational amplifiers are designed with a Darlington output stage. The Darlington configuration, which adds one or more additional transistors, increases the gain of the output stage by a factor of at least xcex2, the common-emitter current gain for a particular transistor. However, the increased gain provided by a conventional Darlington drive is coupled with a reduction in the dynamic output range of the output stage. For each additional transistor in the drive, an additional base-to-emitter voltage difference is introduced from the power supply to the output. This reduction in output range with respect to the power supply increases the power supply voltage required for the drive to function properly.
Thus, conventional Darlington drives may be unable to operate at the lower power supplies which are being used to reduce overall power requirements. Some Darlington drives currently in use have attempted to solve this problem of reduced output range by utilizing a pnpn or npnp compounded approach. However, this approach results in a reduced power-supply rejection ratio, in addition to increasing the complexity and cost of the drive circuit.
In accordance with the present invention, an improved operational amplifier output stage is provided that substantially eliminates or reduces disadvantages and problems associated with previously developed output stages. In particular, the present invention provides a Darlington drive with a relatively high power-supply rejection ratio that is capable of operating with a relatively low power supply.
In one embodiment of the present invention, a system for driving a signal is provided that includes a biasing circuit operable to receive an input signal and operable to produce a bias signal based on the input signal. A drive circuit includes a sensor circuit including a transistor coupled to a first power supply. The drive circuit is coupled to the biasing circuit. The drive circuit is operable to receive the bias signal and to produce an amplified signal based on the bias signal. An output circuit includes a transistor coupled to a second power supply. The output circuit is coupled to the drive circuit. The output circuit is operable to receive the amplified signal and to produce an output signal based on the amplified signal.
Technical advantages of the present invention include providing an improved system and method for driving a signal. In particular, the drive circuit includes a sensor circuit that has a transistor coupled to one power supply, and the output circuit has a transistor coupled to a second power supply. Thus, the Darlington drive of the present invention is referred to the opposite power supply as compared to a conventional Darlington drive. This configuration provides the gain benefits of a conventional Darlington drive without the disadvantages typically associated with Darlington drives. As a result, the invention is able to provide high efficiency and high output drive current, while maintaining a low propagation delay, a high power-supply rejection ratio and a wide dynamic range of the output signal. Additionally, the sourcing and sinking capabilities provided by the present invention are symmetrical. The present invention also provides a double-Darlington drive with the same dynamic output range as a single-Darlington drive.
Other technical advantages will be readily apparent to one skilled in the art from the following figures, description, and claims.